The present invention relates to a radiation image conversion panel that has a phosphor layer such as a stimulable phosphor layer formed by a vapor-phase deposition technique such as vacuum evaporation and achieves excellent sensitivity and sharpness, and a process suitable for producing the radiation image conversion panel.
Upon exposure to a radiation (e.g. X-rays, α-rays, β-rays, γ-rays, electron beams, and ultraviolet rays), certain types of phosphors known in the art accumulate part of the energy of the applied radiation and, in response to subsequent application of exciting light such as visible light, they emit photostimulated luminescence in an amount that is associated with the accumulated energy. Called “storage phosphors” or “stimulable phosphors”, those types of phosphors find use in medical and various other fields.
A known example of such use is a radiation image information recording and reproducing system that employs a radiation image conversion panel having a film (or layer) of the stimulable phosphor (which is hereinafter referred to as a “phosphor layer”). The radiation image conversion panel is hereinafter referred to simply as the “conversion panel” and is also called the stimulable phosphor panel (sheet). The system has already been commercialized by, for example, FUJIFILM Corporation under the trade name of FCR (Fuji Computed Radiography).
In that system, a subject such as a human body is irradiated with X-rays or the like to record radiation image information about the subject on the conversion panel (more specifically, the phosphor layer). After the radiation image information is thus recorded, the conversion panel is scanned two-dimensionally with exciting light to emit photostimulated luminescence which, in turn, is read photoelectrically to yield an image signal. Then, an image reproduced on the basis of the image signal is output as the radiation image of the subject, typically to a display device such as a CRT display or on a recording material such as a photosensitive material.
The conversion panel is typically prepared by the following method: Powder of a stimulable phosphor is dispersed in a solvent containing a binder and other necessary ingredients to make a coating solution, which is applied to a panel-shaped support (substrate) made of glass or a resin, with the applied coating being subsequently dried.
As described in the patent documents to be referred to below, also known are conversion panels which are prepared by forming a phosphor layer on a substrate through vapor-phase deposition techniques (vacuum film deposition techniques) such as vacuum evaporation and sputtering. The phosphor layer formed by such vapor-phase deposition has superior characteristics in that it is formed in vacuo and hence has low impurity levels and that being substantially free of any ingredients other than the stimulable phosphor as exemplified by a binder, the phosphor layer has not only small scatter in performance but also features very highly efficient luminescence.
A phosphor layer formed by vapor-phase deposition may often have a columnar crystal structure formed of columnar phosphor crystals. Various studies and propositions have been made to improve the characteristics of a conversion panel having a phosphor layer formed by vapor-phase deposition.
To be more specific, JP 2003-302498 A refers to manufacture of a conversion panel having a phosphor layer of columnar crystals formed by vapor-phase deposition and discloses that proper control of the temperature of a substrate during the formation of the phosphor layer enables the phosphor layer formed to have a suitable column diameter and to be substantially uniform, whereby the thus obtained conversion panel can be of high image quality.
JP 2004-233067 A discloses a similar conversion panel that has 50 to 4,000 columnar crystals per 100 μm2 of phosphor layer surface to achieve excellent photostimulated luminescence characteristics (luminescence intensity) and high sharpness.
JP 2005-98716 A discloses a similar conversion panel that has a variation coefficient in columnar crystals of up to 50% and preferably up to 40% to achieve high sensitivity and less unevenness in the luminance of the photostimulated luminescence. EP 1359204 A also discloses a similar conversion panel having a variation coefficient in columnar crystals of 0.05 to 0.3.
JP 2004-3955 A discloses a similar conversion panel which has a substrate whose surface is made uneven by a large number of protruding portions and in which columnar crystals are grown only from the protruding portions to be optically isolated from each other, thus achieving high sharpness.